1. Field of the Invention
This invention relates to suspended ceiling grid structures, and more particularly concerns metal inverted T shaped suspended ceiling grid members such as main runners and cross tees and to a method of making them.
2. Description of the Prior Art
Prior art suspended ceiling grid members have been formed by bending a sheet of metal, for example, steel, aluminum, or stainless steel, to form a vertical central web with a bulb at the top and having two web sidewalls and a pair of oppositely disposed horizontal flanges extending outwardly from the bottom of the web to support ceiling tiles. Added strength and rigidity have been given to the inverted T shaped grid member by Ollinger and Shaub in U.S. Pat. No. 4,554,718 by coating the inside portions of the web sidewalls with a hot melt adhesive, and in their U.S. Pat. No. 4,489,529 by fastening together the web sidewalls with hot melt adhesive, a welding, or a lancing pattern.
While these prior art methods of strengthening inverted T grid members have their advantages, the addition of adhesive is somewhat messy and adds to the cost, hot arc welding may destroy a portion of any galvanized coating on both surfaces of the sheet which forms the grid member, and punching lance patterns into the web destroys the integrity of the web.